In general, since a brake system that stops a vehicle that is traveling using hydraulic pressure that increases pedal effort applied to the brake pedal by a driver uses a simple method that holds wheels that are rotating to stop the rotation, it cannot achieve optimum braking performance for traveling conditions of the vehicle and the conditions of the road surface.
In order to overcome limits of the simple brake system as described above, an anti-lock brake system that prevents wheels from locking by appropriately adjusting brake pressure that is applied to the wheels according to a slip ratio that is calculated from the wheels' velocity is used or a traction control system that adjust a driving force of the engine to prevent excessive slip when the vehicle is rapidly started or accelerated is used, in which safety of the vehicle is improved.
However, even in the anti-lock brake system or the traction control system, they can achieve a good performance for a straight road at best, and when the vehicle turns on a curved road, it is impossible to completely control an under steer in which the vehicle is excessively inclined to the outside of the road or an over steer in which the vehicle is excessively inclined to the inside of the road, thereby decreasing reliability of the safety of the vehicle.
For this reason, an EPS (Electronic Stability Program) system, which allows the driver to safely keep the vehicle in a desired direction under any driving condition by minimizing a difference between the traveling direction of the vehicle that the drive wants and the actual traveling direction of the vehicle, in consideration of all of three of braking, driving, and driving systems, that is, by synthetically estimating and controlling the over steer or the under steer in response to measured signals that are inputted from a wheel speed sensor, a yaw rate sensor, a transverse acceleration sensor, and a steering angle sensor, has been used in recent years.
In theses ABS/TCS/ESP systems, separate from a hydraulic (brake) line for supplying hydraulic brake pressure between a master cylinder and the wheels, unexpected turning of the vehicle (e.g. on a ice road) is prevented by pumping oil from the master cylinder and supplying brake pressure to the wheels, in which a solenoid valve is used to provide a oil channel between the master cylinder and a pump.
That is, the solenoid valve is included in a hydraulic circuit in braking, stops the channel or adjusts the increasing speed of hydraulic pressure when the ABS/TCS/ESP is in operation, and operates for rapid release when the braking is released.
However, because the solenoid valve that forms the channel of a liquid pressure line between the master cylinder and the pump should be precisely controlled, the cost is considerably increased by manufacturing of the valve, particularly machining of a valve body that forms the channel and the complicated structure.
Further, because a filter is further needed to filter impurities in the oil, working process and time for assembling the filter is added.